Radio-Frequency (RF) power harvesting techniques have been applied widely for providing long term power supply of low energy consumption and battery-less devices. It is an essential component of passive RFID systems. Recently, there has also been a strong interest in implementing wireless power harvesting techniques with sensor technology for monitoring various environmental conditions such as temperature, pressure, humidity and human health conditions.
Antennas are one of the most critical elements of a RF power harvesting system. The antenna collects ambient radio frequency energy from various sources and delivers the power into the rectifier, which converts the RF power into DC voltage.
An antenna with beam scanning capability is desirable for power harvesting system. By steering the main beam, maximum power could be collected regardless of the position of the power harvesting unit. For some cases, a single antenna is not sufficient for providing reliable power. Therefore, a variety of antenna arrays with different topologies and arrangements have been proposed to increase the overall gain and enhance the efficiency of power harvesting system. Typical antenna arrays mostly improve the efficiency by increasing antenna diversity (multiple bands, multiple rectifiers, DC combiners) or achieve maximum gain at a specific direction (single rectifier, RF combiner), but do not address the issue of beam steering.
When necessary, conventional array antennas use phase shifters to achieve beam steering functionality. However, phase shifters are a poor choice for RF energy harvesting systems, because they increase the cost and complexity of the harvesting system. Phase shifters are not only lossy but also require a significant amount of power to be operable, which would make harvesting power in desirable quantities difficult.